The present invention is directed to conditioning feed material prior to the feed entering a pellet mill, and more particularly, to an annular gap expander to perform the conditioning.
Conditioning refers to adding high levels of mechanical shear and compression to a product, adding high levels of liquids such as fats and molasses to the product, and thoroughly mixing the product. Adding heat to the product is also an advantage because of the reduction of bacteria in the feed. This is important to such operations as poultry production where salmonella can be extremely destructive to the poultry.
In recent years special screw type extruders have been used to highly condition feeds. These units usually consist of a helical conveying screw of rugged construction that has interruptions of the helix evenly positioned along the screw length. Generally at each interruption of the helix along the screw there are positioned radial pins or bars that can be adjusted toward or away from the screw shaft to aid in conditioning. The feed discharges axially at the downstream end of the screw, which is generally not supported by any bearing, resulting in a cantilevered support arrangement for the screw. Resistance to the flow of the feed material through the extruder is produced by a ram that is generally cone shaped and that can be moved axially. The point of the ram is oriented toward the end of the helical screw, and the result is an annular discharge gap between the discharge end of the extruder and the ram. The gap can be adjusted by moving the ram in and out along the axis of the cone.
There are several disadvantages to such an arrangement. First the force of the ram against the flow of the feed translates to the helical screw, therefore increasing the forces on the screw and the bearing arrangement. Since the ram does not generally rotate, the effect of compression on the feed, combined with the high level of liquids in the feed, results in discharging the feed in cohesive lumps. It is usually necessary to install a lump breaker down stream from the extruder to break up any large lumps of conditioned feed. Since only one end of the screw is accessible, it is difficult to directly heat the screw. Other means of heating the feed have included encasing the housing in a heating jacket, increasing the complexity of the extruder.
U.S. Pat. No. 4,097,213 to McComb et al. issued Jun. 27, 1978, is illustrative of another type of prior art extruder apparatus. McComb employs complimentary frustoconically shaped die members to form the annular discharge gap. A female die member is moved axially with respect to a forming die member, which is mounted on the shaft, to adjust the size of discharge gap. Handles, inner and outer annular members, and a sleeve that is internal to the housing are used to move the female die member.
Such a design is impractical for the intended use for several reasons. First, forces developed during adjustment of the gap are imposed on the pressure housing. A large amount of power is required to rotate threaded parts having such large diameters. This power imposes large stresses on the housing which can result in deformation of the housing. A housing designed to withstand such stresses is expensive to manufacture, imposes substantial weight loading on the machine bed, and is difficult to handle during maintenance or repair of the apparatus. Second, such apparatus are effectively limited in size. The parts required to construct a larger mechanism of this design are prohibitively expensive to manufacture. As the apparatus size increases the power required to rotate the threaded parts becomes excessive. The additional power imposes additional stress on the housing. As a practical matter, the power requirements and stresses developed effectively limit the size of such apparatus. Third, the discharge gap of such apparatus cannot be adjusted in a timely manner during operation of the apparatus. The gap is adjusted manually and the operator must overcome the pressure developed in the apparatus. This effectively limits the operator's ability to adjust the discharge gap in a timely manner.